Method of measuring bone strain to detect fracture consolidation

ABSTRACT

A surgical method of measuring bone strain to detect fracture consolidation includes the steps of affixing an external fixation device to the patient&#39;s bone that has a fracture site to be healed and wherein the fixation device supports an elongated optical fiber. Light is transmitted through the optical fiber to an exit point. The light intensity within the optical fiber is measured, preferably at the exit point as the light travels through the optical fiber. Light intensity is measured to monitor strain in the fixation device as the fracture site heals. A physician then determines when the site has fully healed by observing changes in light intensity as the patient&#39;s bone rather than the fixation device carries more of the patient&#39;s normal body loading.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to surgical orthopedic instrumentation andto the external fixation of bone fractures. More particularly, thepresent invention relates to an improved method and apparatus fordetermining the extent of healing of a bone fracture that is supportedduring the healing process by an external fixation device. Even moreparticularly, the present invention relates to an improved method andapparatus for measuring bone strain after a bone has been fractured andis supported during the healing process with a surgically attachedexternal fixation and wherein an external fixation device carries one ormore light transmitting elements (such as an optical fiber), and lightintensity transmitted through the optical fiber is measured to determinewhether or not the patient's bone has fully healed to carry normalloading generated by the patient (such as the weight of the patient).

2. General Background

One of the major questions presented to a surgeon after treating a bonedeformity or problem fracture is the question of bone healing.Specifically, the surgeon needs to know whether or not the healing iscomplete. Physicians are frequently presented with the question of whencan a cast, plate, screw or external fixation device be removed. If thedoctor continues treatment longer than necessary, the patient can besubjected to undue discomfort. If the doctor prematurely removes such adevice, there is a risk of refracture.

Currently, physicians use an x-ray of the fracture site to make adecision regarding the extent of healing of a bone fracture. However,x-rays do not always give a clear and accurate assessment. Oftenphysicians are forced to make judgment calls regarding the degree ofhealing at a fracture site.

External fixation devices for setting bone fractures are known in theart. One example is U.S. Pat. No. 4,576,158 entitled "Method ForDetermining Stability of an Orthopedic Device Composed of an ExternalFixation Bar During Setting of Bone Fractures", issued to MichaelBoland. In the Boland patent, the stability of an orthopedic assemblycharacterized by external fixation means in the form of a fixation barprovided with five assemblies of strain gauges having at least one gaugeeach, involves a measuring of the amount of torsion of the bar by one ofthe assemblies and measuring of two perpendicular components of theamount of bending of the bar at two points of the bar via the remainingtwo pairs of assemblies and determining by means of the strain gaugesthe amount of torsion and the amount of bending at the level of theclamps which connect the fixation bar to the pins and by comparison ofthe determined levels of the torsion and bending with those capable ofcausing sliding of the clamps, the extent of stability of the orthopedicassembly.

Patents have been issued which describe the use of fiber optics as partof a strain gauge. These include U.S. Pat. No. 4,191,470 issued to C. D.Butter and U.S. Pat. No. 4,841,778 issued to M. A. Butler. The Butterpatent is directed to a laser-fiber optic interferometric strain gauge.This is a strain gauge that can be read out by means of fiber optics,which has no power other than optical at the strain gauge. Laser lightis directed into two single mode fibers, which fibers are attached to asupporting member, the strain gauge of which is to be measured. The endsof the fibers are brought into close proximity and the light coming outof the two fibers interferes. When the supporting member is strained,the fringes move giving a measure of the strain.

In the Butler patent, (U.S. Pat. No. 4,841,778), laser light from acommon source is split and conveyed through two similar optical fibersand emitted at their respective ends to form an interference pattern,one of the optical fibers having a portion thereof subjected to astrain. Changes in the strain cause changes in the optical path lengthof the strain fiber, and generate corresponding changes in theinterference pattern. The interference pattern is received andtransduced into signals representative of fringe shifts corresponding tochanges in the strain experienced by the strained one of the opticalfibers. These signals are then processed to evaluate strain as afunction of time. Typical examples of the application of the apparatusinclude electrodeposition of a metallic film on a conductive surfaceprovided on the outside of the optical fiber being strained, so thatstrains generated in the optical fiber during the course of theelectrodeposition are measurable as a function of time. In one aspect ofthe invention, signals relating to the fringe shift are stored forsubsequent processing and analysis, whereas in another aspect of theinvention the signals are processed for real-time display of the strainchanges under study.

SUMMARY OF THE PRESENT INVENTION

The present invention provides a doctor with a diagnostic tool to aid inthe assessment of the degree of healing of a fractured or injured bone.The present invention will indirectly measure the strain experienced bya bone and provide feed-back to the doctor.

The present invention includes one or more optical fibers embedded intoa preferably composite material that is used as a supporting member ofan orthopedic external fixation device.

The optical fiber can have a preferably plug type connector at each end,protruding from the composite material. At one end of the optical fiber,a laser can be attached. The light emitted by the laser can betransmitted through the embedded optical fiber to the opposite endportion thereof where a photoelectric sensor or the like can be attachedto measure the light intensity. The amount of light intensity willcorrespond to the amount of strain experienced by the compositematerial.

An example of an application of this technique would be an optical fiberembedded into a hexagonally shaped bar of composite material thatsupports pins that are affixed to the patient's bone above and below afracture site or osteotomy site. At the beginning of the treatment, allthe weight of the patient is transferred from the bone through the pins,to the composite hex bar and back to the bone thus bypassing thefracture site.

As the doctor connects the diagnostic equipment and measures the laserlight, initially he sees a low light intensity because the strain in thehex bar bends the fiber optic strand and diffuses the laser light.

After the bone has healed, the weight of the patient transfers throughthe bone instead of the hex bar. The light intensity is then brighterbecause the optical fiber would not be deformed. The doctor would thenknow the ideal time to remove the fixation device, because the doctorwould know the light transmission value for the undeformed opticalfiber.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the nature and objects of the presentinvention, reference should be had to the following detailed descriptiontaken in conjunction with the accompanying drawings, in which like partsare given like reference numerals, and wherein:

FIG. 1 is a schematic elevational view of the first and preferredembodiment of the apparatus of the present invention;

FIG. 2 is a side fragmentary view of the first embodiment of theapparatus of the present invention illustrating the composite barportion of the external fixation device;

FIG. 3 is a sectional fragmentary view of the first embodiment of theapparatus of the present invention, illustrating the composite barportion of the external fixation device;

FIG. 4 is a schematic view of a second embodiment of the apparatus ofthe present invention;

FIG. 5 is a partial perspective view of a third embodiment of theapparatus of the present invention;

FIG. 6 is a schematic view of a fourth embodiment of the apparatus ofthe present invention;

FIG. 7 is a schematic view of a fifth embodiment of the apparatus of thepresent invention;

FIG. 8 is a schematic view of a sixth embodiment of the apparatus of thepresent invention;

FIG. 9 is a schematic view of a seventh embodiment of the apparatus ofthe present invention;

FIG. 10 is a schematic view of an eighth embodiment of the apparatus ofthe present invention;

FIGS. 11-12 are schematic elevational views of a ninth embodiment of theapparatus of the present invention; and

FIG. 13 is an schematic view of a tenth embodiment of the apparatus ofthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1-3 show the preferred embodiment of the apparatus of the presentdesignated generally by the numeral 10. In FIG. 1 bone strain monitor 10is attached to a patient's femur 11 that has been fractured, thefracture site designated as 12 in FIG. 1.

The patient's femur 11 has a proximal end 13 and a distal end portion14. A plurality of pins 15-17 are attached to one side of the femurabove the fracture 12 and adjacent proximal end 13 as shown in FIG. 1. Aplurality of bone pins 18-20 are attached to the portion of femur 11adjacent distal end 14 and below fracture 12. Each of the pins 15-20attaches to the fixation bar 21 that is shown more particularly in FIGS.2 and 3.

Bar 21 is preferably elongated, having a proximal end 21 and a distalend 23. A laser attaches to bar 21 at 24 in such a fashion that thelaser beam projects into bar 21 along a line that aligns with fiberoptic cable 27. The fiber optic cable 27 has end portions 29, one ofwhich end portions 29 receives laser beam 24. The opposite end portionof fiber optic cable 27 connects to output cable 26 that can beinterfaced with a data recorder (commercially available from Newport).The data recorder (not shown) records light intensity of the laser beamas it exits and end portion 29 of fiber optic cable 27.

Bar 21 can have a hexagonal outer surface 28. Though one cable 27 isshown in FIGS. 2-3, a plurality of fiber optic cables 27 could be placedwithin bar 21. For example, six fiber optic cables could be positionedabout the periphery of bar 21, arranged in a regular pattern, equallyspaced both radially and circumferentially. Preferably, a plurality ofcables are positioned at the center and near the periphery of bar 21.

The remaining embodiments of FIGS. 4-13 illustrate other configurationsfor measuring bone strain as transmitted through a pair of bone pins. InFIG. 4, designated generally by the numeral 30, a pair of pins 31, 32are shown. Each of these pins 31, 32 would be embedded in the patient'sfemur 11, respectively above and below the fracture 12. The pin 31receives laser beam 33 at right angles thereto. The beam 33 travelsalong line 35 to reflective surface 34.

Depending upon the angular displacement of pin 32 relative to pin 31,the laser reflects angularly along line 36 as shown in FIG. 4. The beams35 and 36 define an angle 37 therebetween. Graduations 38 are placedalong pin 31, each of the graduations defining a load value thatcorresponds to the load being carried by the fixation bar 21 and thus bythe bone 11. When the angle 37 is minimized, the beams 35 and 36 alignmeaning the patient's bone has healed and is carrying all of the load.

In FIG. 5, the bone strain monitor is designated generally by thenumeral 40. In FIG. 5, a bar 41 having a hexagonal out of surface 42carries an outer photoelastic coating 43. The photoelastic coating 43allows the user to view strain lines on the coating and to photographsame if desired. When the photoelastic coating does not show strainlines, the bar no longer carries load, the load being carried by thepatient's bone indicating that healing has been completed.

FIGS. 6-7 and 13 show other variations of the photoelastic coating aspart of a bone strain monitor. In the embodiment of FIG. 6, designatedgenerally by the numeral 44, a pair of pins 45, 46 are shown connectedto a U-shaped member 47. The U-shaped member attaches at 48 and 49 tothe pins 45, 46 respectively. A photoelastic coating is placed at 50 fordemonstrating when the pins are deflected relative to one another thusdemonstrating that the bar is carrying load. When the photoelasticcoating 50 shows that no strain is being carried by the bar, thepatient's bone has healed and is carrying all the load.

In the embodiment of FIG. 7, designated generally by the numeral 51, apair of pins 52, 53 are shown, each connecting to a support member 54,55 respectively. The support members 54, 55 attach to plate 56 that hasa photoelastic coating thereon. The plate can have a gap 57 and aperiphery 59. This configuration creates easily viewed and photographedstrain lines 58 that amplify an load carried by the bar as opposed tothe patient's bone. When the strain lines 58 disappear, the patient'sbone has healed and is carrying all of the load.

In the embodiments of FIGS. 8 and 9 (designate respectively by numerals60 and 64) photoelastic members are used. In FIG. 8, a pair of pins 61,62 attach to a photoelastic clear plastic rod 63. In FIG. 9, pins 65, 66attach to member 67, 68 that support plate 69. The plate 69 has a recessthat is generally V-shaped at 70 and a periphery 71. The strain lines 72are shown extending away from the V-shaped portion 70.

In FIG. 10, designated generally by the numeral 73, a linear variabledifferential transducer is shown. If there is a small gap at thefracture site 12, the pins will bend toward the horizontal axis passingthrough the fracture site, which puts the hexagonal bar in constantbending moment.

Linear variable differential transducer placed near the soft tissue canmeasure accurately how much the distance between the pins is shortened.This measurement can correspond to the amount of load that is beingtransferred to the pins by a pair of segments that support the linearvariable differential transducer. The reduction of the gap closer meansthat the loads are decreasing when the pins and thus the bone iscarrying the load and healing as progressing. In FIG. 13 a pair of pins73, 75 are shown attached to femur 11 above and below fracture site 12.Each of the pins 74, 75 supports an arm 76, 77. The ends 81, 82 of thearms 76, 77 translate toward and away from each other as the pins carryload. The linear variable differential transducer 83 extends between theend portions 81, 82. A gap exists between the end portions 81, 82. Thereduction of the gap means that loads are decreasing on the pins, thusthe bone 11 is carrying the load and healing is progressing. Otherembodiment that employ a linear variable differential transducer areshown in FIGS. 11 (designated by the numeral 84) and FIG. 12 (designatedby the numeral 92).

In FIG. 11, the femur 11 and fracture site 12 are shown. A pair of pins85, 86 are disposed adjacent fracture 12 but above and below fracture 12respectively. A second pair of pins 87, 88 are spaced further away fromthe fracture site 12 and above and below the fracture site respectively.Linear variable differential transducer 90 includes a gap portion 91.The pins 85-88 are attached to bar 89 as shown.

In the embodiment of FIG. 12 designated by the numeral 92, the femur 11supports pins 93-96 above and below the gap as shown in the drawing.Each of the pins 93-96 is also attached to the bar 99. The linearvariable differential transducer 97 is generally L-shaped, being rigidlyattached to the pin 93 and including a gap 98. Each of the embodimentsof FIGS. 11 and 12, reduction of the gap means that loads are decreasingon the pins, thus the bone is carrying the loads and healing isprogressing.

FIG. 13 illustrates a further embodiment designated generally by thenumeral 100. In FIG. 13, a pair of pins 101, 102 are attached to member103 having a circular opening 104. The member 103 carries a photoelasticcoating that shows strain lines if the pins 101, 102 carry bendingmoment thus indicating that the bone is transmitting the load to themember 103. The user can observe the coating under polarized light toobserve the strain pattern display.

The following table lists the part numbers and part descriptions as usedherein and in the drawings attached hereto.

    ______________________________________                                        PARTS LIST                                                                    Part Number      Description                                                  ______________________________________                                        10               bone strain monitor                                          11               femur                                                        12               fracture                                                     13               proximal femur                                               14               distal femur                                                 15               pin                                                          16               pin                                                          17               pin                                                          18               pin                                                          19               pin                                                          20               pin                                                          21               bar                                                          22               proximal end                                                 23               distal end                                                   24               laser                                                        25               light sensor                                                 26               output cable                                                 27               fiber optic cable                                            28               hexagonal outer surface                                      29               end portion                                                  30               bone strain monitor                                          31               pin                                                          32               pin                                                          33               laser beam                                                   34               reflecting surface                                           35               first line                                                   36               second line                                                  37               angle                                                        38               graduation                                                   39               maximum deflection                                           40               bone strain monitor                                          41               bar                                                          42               hexagonal outer surface                                      43               photoelastic coating                                         44               bone strain monitor                                          45               pin                                                          46               pin                                                          47               U-shaped member                                              48               attachment                                                   49               attachment                                                   50               photoelastic coating                                         51               bone strain monitor                                          52               pin                                                          53               pin                                                          54               support                                                      55               support                                                      56               plate                                                        57               recess                                                       58               strain lines                                                 59               periphery                                                    60               bone strain monitor                                          61               pin                                                          62               pin                                                          63               rod                                                          64               bone strain monitor                                          65               pin                                                          66               pin                                                          67               support                                                      68               support                                                      69               plate                                                        70               recess                                                       71               periphery                                                    72               strain lines                                                 73               bone strain monitor                                          74               pin                                                          75               pin                                                          76               support                                                      77               support                                                      78               slot                                                         79               slot                                                         80               pin                                                          81               end                                                          82               end                                                          83               transducer                                                   84               bone strain monitor                                          85               pin                                                          86               pin                                                          87               pin                                                          88               pin                                                          89               bar                                                          90               transducer                                                   91               gap                                                          92               bone strain monitor                                          93               pin                                                          94               pin                                                          95               pin                                                          96               pin                                                          97               transducer                                                   98               gap                                                          99               bar                                                          100              bone strain monitor                                          101              pin                                                          102              pin                                                          103              photoelastic member                                          104              circular opening                                             ______________________________________                                    

Because many varying and different embodiments may be made within thescope of the inventive concept herein taught, and because manymodifications may be made in the embodiments herein detailed inaccordance with the descriptive requirement of the law, it is to beunderstood that the details herein are to be interpreted as illustrativeand not in a limiting sense.

What is claimed as invention is:
 1. A surgical method of measuring bonestrain to detect fracture consolidation, comprising the steps of:a)affixing an external fixation device to a patient's bone that has afracture site to be healed, and wherein the fixation device supports anelongated optical fiber cable; b) transmitting light through the opticalfiber cable to an exit point; c) measuring the light intensity thattravels through the optical fiber cable; d) using the light intensitymeasurement to monitor strain in the fixation device, as the fracturesite heals; and e) determining when the fracture site has healed byobserving changes in light intensity as the patient's bone rather thanthe fixation device carries the patient's normal body loading.
 2. Thesurgical method of claim 1 wherein in step "a" the fixation device is ofa composite material and further comprising embedding the optical fibercable within the fixation device.
 3. The surgical method of claim 1wherein in step "a" the fixation device is an elongated bar having acentral longitudinal axis, and the elongated optical fiber is a linearstructure that is positioned within the bar and generally aligned withthe central longitudinal axis.
 4. The surgical method of claim 1 whereinin step "d" the light intensity is measured at one end portion of theoptical fiber.
 5. The surgical method of claim 1 wherein the opticalfiber bends when the external fixation device is loaded to support thebone.
 6. The surgical method of claim 1 wherein in step "c" aphotoelectric sensor is used to measure the light intensity.
 7. Asurgical method of measuring bone strain to detect fractureconsolidation, comprising the steps of:a) affixing an external fixationdevice to a patient's bone that has a fracture site to be healed, andwherein the fixation device supports an elongated optical fiber; b)transmitting light through the optical fiber to an exit point; c)measuring the light intensity that travels through the optical fiber; d)using the light intensity measurement to monitor strain in the fixationdevice, as the fracture site heals; e) determining when the fracturesite has healed by observing changes in light intensity as the patient'sbone rather than the fixation device carries the patient's normal bodyloading; and f) wherein in step "b" a laser is used to transmit lightthrough the optical fiber.
 8. The surgical method of claim 1 wherein instep "d" the amount of light intensity correspond to the amount ofstrain experienced by the fixation device.
 9. The surgical method ofclaim 1 wherein in step "a" the fixation device is attached to thepatient's bone above and below the fracture or osteotomy site.
 10. Thesurgical method of claim 9 wherein the fixation device is an elongatedbar affixed to the patient's bone with pins.
 11. The surgical method ofclaim 1 wherein in step "d" the light transmitted is preliminarilydiffused because the fiber optic strain and fixation device are bent.12. The surgical method of claim 10 wherein the pins are preliminarilysurgically implanted.
 13. The surgical method of claim 1 wherein thereare a plurality of optical fibers carried by the external fixationdevice.
 14. A surgical method of measuring strain in an external fixatorthat supports a patient's bone that has been fractured comprising thesteps of:a) affixing an external fixation device to a patient's bonethat has a fracture site to be healed, and wherein the fixation deviceattaches to the bone at spaced apart positions proximally and distallyof the fracture site and the external fixation device having an outersurface that displays on said outer surface a visible indicator of thestrain within the fixation device; b) measuring the deflection of theexternal fixation device; c) determining when the fracture site hashealed by observing changes in deflection of the external fixationdevice as the patient's bone heals and until the patient's bone ratherthan the fixation device carries the patient's normal body loading. 15.A surgical method of measuring strain in an external fixator thatsupports a patient's bone that has been fractured comprising the stepsof:a) affixing an external fixation device to a patient's bone that hasa fracture site to be healed, wherein the fixation device attaches tothe bone at spaced apart positions proximally and distally of thefracture site and the strain is measured with a sensor embedded withinthe fixation device that gives a visible indicator of the strain withinthe fixation device; b) measuring the deflection of the externalfixation device and its embedded sensor; c) determining when thefracture site has healed by observing changes in deflection of theexternal fixation device and embedded sensor as the patient's bone healsand until the patient's bone rather than the fixation device carries thepatient's normal body loading; and d) visualizing the fixation device toobserve deflection and wherein the fixation device has a photoelasticcoating that evidences deflection.
 16. The surgical method of claim 15further comprising the step of placing a pair of fixation pins above andbelow the fracture, and the fixation device includes a member that spansbetween the pins.
 17. The surgical method of claim 15 further comprisingthe step of continuously comparing how much of the flexural load of thefixation device is being reduced by the patient's bone sharing in acarrying of the anatomical loading.
 18. The surgical method of claim 15further comprising the step of observing the coating under polarizedlight to observe the strain pattern display.
 19. The surgical method ofclaim 18 further comprising the step of photographing the coating andstrain pattern display.
 20. A surgical method of measuring strain in anexternal fixator that supports a patient's bone that has been fracturedcomprising the steps of:a) affixing an external fixation device to apatient's bone that has a fracture site to be healed, and wherein thefixation device supports a visible indicator of strain within thefixation device; b) measuring the deflection of the external fixationdevice; c) determining when the fracture site has healed by observingchanges in deflection of the external fixation device as the patient'sbone heals and until the patient's bone rather than the fixation devicecarries the patient's normal body loading; and d) transmitting a laserbeam from one end of the fixation device to the other end of thefixation device and reflecting the laser beam with a reflecting memberand measuring the angle of reflection.
 21. The surgical method of claim15 wherein the fixation device includes a pair of spaced apart bone pinsattached to spaced apart positions on the patient's bone and a memberspanning the pins and further comprising the step of observing thephotoelastic coating to determine strain distribution on the member.